President-elect Joe Biden’s choice to run the Centers for Disease Control and Prevention is a widely respected infectious-diseases specialist regarded as a strong communicator unafraid to speak her mind, qualities critical to returning the beleaguered public health agency to its traditional front-line role and to bringing the coronavirus pandemic under control.

But while Rochelle Walensky’s research has long had a public health focus, she has never run a government agency or organization as large and complex as the CDC.

Walensky, 51, heads the infectious-diseases department at Massachusetts General Hospital, one of the nation’s storied medical centers, and is a professor of medicine at Harvard Medical School. She has conducted pioneering research on HIV and AIDS, with an emphasis on equity and access to treatment.

Her research has included topics such as the effectiveness of treatment in stopping HIV’s spread and cardiovascular disease among people receiving HIV medicine in South Africa.

The challenges in running the CDC are enormous. It was once the most admired public health agency in the world but has experienced a loss of institutional credibility during the Trump administration.

The agency has endured false accusations and interference by Trump political appointees, who sidelined the agency early in the pandemic. Political appointees delayed or ordered revisions to critical CDC guidance, including the benefits of masks, reopening churches and testing of asymptomatic individuals, officials have said. Political appointees have also sought to exert control over scientists and health officials’ messages, including the content of CDC’s weekly science reports.

Walensky posted a brief message on Twitter on Monday: “I began my medical career at the height of the HIV/AIDS crisis, and I’ve spent my life ever since working to research, treat, and combat infectious diseases. I’m honored to be called to lead the brilliant team at the CDC. We are ready to combat this virus with science and facts.”

The Advisory Committee on Immunization Practices voted on the recommendations for whom should be given the covid-19 vaccine first when it becomes available. (The Washington Post)

On Tuesday, after Biden officially introduced her as part of his health team during an event in Wilmington, Del., she described how her medical training more than 30 years ago coincided with some of the most harrowing years of the AIDS crisis.

“Inside the hospital, I witnessed people lose strength and hope, while outside the hospital, I witnessed those same patients, mostly gay men and members of vulnerable communities, be stigmatized and marginalized by their nation and many of its leaders,” she said.

She has dedicated her career since to researching and treating infectious diseases and to ending the AIDS crisis, she said.

“Now. a new virus is ravaging us. It’s striking hardest once again at the most vulnerable, the marginalized, the underserved,” she said. “We are losing life and hope at an alarming rate.”

While government service had never been part of her plan, she likened the pandemic to a national medical emergency. “When the nation is coding … you run to take care of people to stop the bleeding, to stabilize, to give them hope and a fighting chance to come back stronger,” she said.

“To the American people and to each and every one of you at the CDC, I promise to work with you, to harness the power of American science to fight this virus and prevent unnecessary illness and deaths so that we can all get back to our lives.”

With the country poised to embark on an unprecedented vaccination campaign against the novel coronavirus, the CDC is among the agencies leading that effort, which includes persuading people to get shots.

With a budget of nearly $8 billion, the nation’s chief public health agency has many responsibilities, including investigating disease outbreaks and figuring out how best to prevent the leading causes of death in the United States, such as heart disease and cancer.

Carlos del Rio, an infectious-diseases physician at Emory University School of Medicine, described Walensky’s lack of experience running a sprawling public health agency as “a big challenge but not insurmountable.” Del Rio has known Walensky for decades and described her as a “fantastic researcher, a terrific scientist, someone driven by data.” He and others said Walensky will face a steep learning curve and will need many people at the Atlanta-based agency to support her.

Del Rio likened the task to turning around a giant oil tanker that hasn’t yet sunk “but has a major hole in its bow, and you have to cover the hole, you have to redirect the tanker, and you have to paint the tanker.”

Experts said the early announcement of a CDC director, whose appointment does not require Senate confirmation, took many by surprise. In previous administrations, it has taken months to make the appointment. Walensky’s naming signals the position’s primacy and that “science and evidence-based policy will guide one of the most important agencies, if not the most important, in getting the country through the pandemic,” said Kavita Patel, a physician who served in the Obama White House as a policy director and is a nonresident fellow at the Brookings Institution.

Walensky will replace Robert Redfield, who was appointed in April 2018. In a September Washington Post article, many inside and outside the agency blamed Redfield for weak leadership and a failure to protect CDC’s career staff and science-based guidance from political meddling by the White House and Department of Health and Human Services officials. Morale at the agency is at an all-time low, career scientists have told The Post in recent months.

In a statement at the time, Redfield pledged to “do everything in my power” to support the CDC’s scientists.

Experts said Walensky’s priorities should include unmuzzling CDC experts — including herself — and allowing them to speak directly and regularly to the public.

“You need to have daily or three-times-a-week briefings and lay out, ‘This is where we are, and these are what the challenges are,’ ” del Rio said.

Some public health experts worry that a singular focus on infectious diseases could lead to neglecting chronic diseases, which account for a huge and growing proportion of the country’s long-term health-care costs.

But those who have worked closely with Walensky said they have confidence in her ability to learn and adjust.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, who will be a top medical adviser in the Biden administration, called her a valued colleague and friend.

“She is an outstanding individual, highly respected in her field of infectious diseases, who I have no doubt will adjust and adapt to the challenges involved in managing a large public health agency,” Fauci said.

News of her appointment late Sunday was greeted with almost universal exuberance among fellow infectious-diseases experts, who shared it widely on social media. “This news has sent me into a sort of public health euphoria,” wrote Julia Marcus, a Harvard colleague. “It gives me such hope to know that someone as brilliant, kind, and fearless as @Rwalensky will lead restoration of our preeminent public health agency.”

Within the CDC, the news was also welcome, although some officials, speaking on the condition of anonymity, were taken aback.

“I was hoping for someone who is an experienced public health leader, but hopefully she’ll become one quickly. I hope whoever sits in those positions will consider ways to change governance to limit political influence in the future,” said one CDC epidemiologist who spoke on the condition of anonymity to share personal thoughts.

Another CDC official, speaking on similar terms, said the agency has other people who can provide “complementary” skills. A third senior official who has worked with Walensky said she would bring “a breath of fresh air” and said she has the capabilities “to be one of the best CDC directors in the history of the agency.”

An official close to the Biden transition who spoke on the condition of anonymity to discuss the pick said Walensky has “unparalleled credibility on infectious diseases and brings fresh leadership.” The official added: “There are plenty of experienced managers at CDC she can rely on.”

At Massachusetts General, Walensky oversees more than 70 physicians.

“She is not afraid to stand up to power and be honest about her opinions,” hospital President Peter L. Slavin said. Earlier in the pandemic, before the benefits of mask-wearing were clearly communicated and concern loomed about a shortage of masks, “she pushed the hospital really hard to make everyone wear masks, that it would stop the spread among staff and potentially save lives,” he said. Within days, the hospital followed her advice, he said.

Others, including Angela Rasmussen, a virologist at the Georgetown Center for Global Health Science and Security, cite Walensky’s leadership in condemning a controversial proposal to defeat the virus by cloistering the most vulnerable and allowing the illness to spread and burn out among the rest of the population.

Walensky grew up in Potomac, Md., and received her medical degree from Johns Hopkins University. She has a master’s in public health from Harvard’s T.H. Chan School of Public Health. Her husband, Loren Walensky, is a pediatric oncologist at Dana-Farber Cancer Institute and Boston Children’s Hospital. They have three sons, ages 16, 18 and 21.

Alice Crites contributed to this report.